Abstract. The complexity of dissolved gas cycling in the ocean presents a
challenge for mechanistic understanding and can hinder model intercomparison.
One helpful approach is the conceptualization of dissolved gases as the sum
of multiple, strictly defined components. Here we decompose dissolved
inorganic carbon (DIC) into four components: saturation
(DICsat), disequilibrium (DICdis), carbonate
(DICcarb), and soft tissue (DICsoft). The cycling
of dissolved oxygen is simpler, but can still be aided by considering
O2, O2sat, and O2dis. We explore changes in
these components within a large suite of simulations with a complex coupled
climate–biogeochemical model, driven by changes in astronomical parameters,
ice sheets, and radiative forcing, in order to explore the potential
importance of the different components to ocean carbon storage on long
timescales. We find that both DICsoft and DICdis
vary over a range of 40 µmol kg−1 in response to the climate
forcing, equivalent to changes in atmospheric pCO2 on the
order of 50 ppm for each. The most extreme values occur at the
coldest and intermediate climate states. We also find significant changes in
O2 disequilibrium, with large increases under cold climate states. We
find that, despite the broad range of climate states represented, changes in
global DICsoft can be quantitatively approximated by the
product of deep ocean ideal age and the global export production flux. In
contrast, global DICdis is dominantly controlled by the
fraction of the ocean filled by Antarctic Bottom Water (AABW). Because the
AABW fraction and ideal age are inversely correlated among the simulations,
DICdis and DICsoft are also inversely correlated,
dampening the overall changes in DIC. This inverse correlation could be
decoupled if changes in deep ocean mixing were to alter ideal age
independently of AABW fraction, or if independent ecosystem changes were to
alter export and remineralization, thereby modifying DICsoft.
As an example of the latter, we show that iron fertilization causes both
DICsoft and DICdis to increase and that the
relationship between these two components depends on the climate state. We
propose a simple framework to consider the global contribution of
DICsoft+DICdis to ocean carbon storage
as a function of the surface preformed nitrate and DICdis of
dense water formation regions, the global volume fractions ventilated by
these regions, and the global nitrate inventory.